This invention relates to the modulation of laser beams, and more particularly it relates to a laser beam modulator using Stark-tuned absorption resonances.
In recent years there has been considerable interest in developing laser beam modulators utilizing the Stark effect (the change in the spectrum of a molecule when subjected to an electric field). A laser beam is passed through a cell containing gas molecules having an absorption resonance near the frequency of the laser beam, and a dc electric field is applied to the cell to shift, or turn, the absorption resonance so as to overlap the frequency of the laser beam. A modulating signal applied to the cell varies the absorption resonance about its steady state value, producing modulation of the laser beam.
Some of the early work in Stark-effect laser beam modulation is described in a paper by A. Landman et al, "Light Modulation by Means of the Stark Effect in Molecular Gases-Application to CO.sub.2 Lasers", Applied Physics Letters, Vol. 15, No. 11 (Dec. 1, 1969), pages 357-360. These modulators employed Stark cells approximately 1 meter in length and containing a number of different absorbing gases at pressures ranging from about 2 to about 10 Torr. The highest modulation depth (the ratio of the amplitude of the modulation envelope to the carrier amplitude) achieved with such modulators was 20% at a modulating frequency of about 250 Hz.
Following the initial work with Stark cell laser modulators, substantial effort was made to achieve greater modulation depths, higher modulating frequencies, and shorter Stark cells. This effort has involved, among other things, a search for new combinations of particular laser lines and Stark-tunable absorbing materials. As a result of this effort a number of Stark-effect modulators were developed.
One such modulator is disclosed in U.S. Pat. No. 3,806,834 to A. R. Johnston et al. In this modulator the 10.6 .mu.m, P(20) line of a C.sup.12 O.sub.2.sup.16 laser is used in conjunction with a Stark cell filled with about equal parts of N.sup.14 H.sub.3 and N.sup.14 D.sub.3 (to form N.sup.14 H.sub.2 D). A modulation depth of 40% was achieved for a modulating frequency of 400 Hz in a cell about 20 cm long with a gas pressure of 4 Torr.
Another Stark-effect modulator is disclosed in a paper by J. T. LaTourrette et al, "An Efficient Stark-Effect Modulator at 9.6 Microns", Symposium on Optical and Acoustical Micro-Electronics, Polytechnic Press of the Polytechnical Institute, Brooklyn, N.Y., 1975, pages 535-541. This modulator utilized the P(32) rotational line (at 1035.47 cm.sup.-1) of the (001-020) vibrational band of a C.sup.12 O.sub.2.sup.16 laser in conjunction with a Stark cell filled with the methyl fluoride isotopic species C.sup.13 H.sub.3 F. A modulation depth of 10% was achieved with audio frequency modulation (e.g., 60 Hz) in a cell of 10 cm length, and a relatively constant modulation response was obtained as the frequency was increased to about 30 MHz (with a reduced modulation depth). However, due to saturation of the absorbing medium, the maximum usable laser power density was less than 0.1 watt per cm.sup.2.
One searching for new combinations of particular laser lines and Stark-turnable absorbing materials has available to him an abundance of spectroscopic data with respect to materials having absorption resonances at frequencies in the vicinity of known laser transition frequencies. For example, in a paper by F. Shimizu "Stark Spectroscopy of NH.sub.3 .nu..sub.2 Band by 10-.mu.CO.sub.2 and N.sub.2 O Lasers", Journal of Chemical Physics, Vol. 52, No. 7 (Apr. 1, 1970), pages 3572-3576, an extensive table is presented listing near coincidences between N.sup.14 H.sub.3 absorption lines and various C.sup.12 O.sub.2.sup.16 laser lines. In addition, in a paper by F. Allario et al, "Measurements of NH.sub.3 Absorption Coefficients with a C.sup.13 O.sub.2.sup.16 Laser", Applied Optics, Vol. 14, No. 9 (September 1975), pages 2229-2233, measured absorption coefficients of N.sup.14 H.sub.3 are given using the transitions R(8) through R(28) of a C.sup.13 O.sub.2.sup.16 laser. The absorption measurements were made with N.sup.14 H.sub.3 gas at a pressure of 1 Torr broadened to a total pressure of 760 Torr by the addition of N.sub.2 gas and in the absence of any electric field.
With the aforementioned prior art at hand, attempts to develop improved Stark-effect laser modulators continued with the goals of increasing the modulation depth, increasing the workable modulating frequencies, increasing the usable laser power density, and at the same time providing a reliable device of minimum size and weight which could be made as inexpensively as possible. However, since many of these goals are incompatible with one another, it seemed that the realization of a Stark-effect modulator in which all of these goals could be achieved at the same time was beyond the grasp of the scientific community.